This invention relates to the treatment of a hygroscopic material such as tea or tobacco. Such treatments are carried out, for example, with the intention of increasing the materials pliability by the introduction of moisture and heat into the material or with the intention of introducing cellular expansion. The introduction of pliability is advantageous since it reduces the material's fragility and the material becomes better able to resist mechanical damage in subsequent handling. The introduction of cellular expansion is advantageous for products made from the material where a principle judgement criteria is minimisation of the mass of material required to occupy a given volume. The relevancy of the invention can be illustrated by reference to tobacco processing.
It is well known that moisture penetration into the structure of a hygroscopic material requires a heat energy input known as the energy of moisture adsorption. This energy may be derived from the surrounding environment gradually with time, or more quickly by passing steam through the material to provide both heat and moisture.
It is well known that hygroscopic organic materials such as tobacco are thermally sensitive and that their exposure to heat will introduce chemical change and related changes in their physical properties. In particular heating of the material, while inducing temporary pliability to the product while it is at elevated temperature, will also induce chemical change so that when the material cools and loses it's temporary pliability, it's pliability at normal temperature and moisture is actually less that it was prior to the heating operation. Further the higher the temperature the material is subjected to, the less pliable and more fragile it becomes when it reverts to normal temperatures.
This is illustrated below, which shows the effect of average tobacco temperatures as it exits from an expansion process and the quantity of small particles in the tobacco after it has been reduced to normal temperature and moisture by a subsequent drying process.
______________________________________ Tobacco Concentration of Tobacco Average temperature Small Particles after at Exit from Expansion Subsequent Drying Process % Process .degree.C. below 1 mm ______________________________________ 94 8.0 102 8.5 104 11.4 ______________________________________
The results indicate that as the expansion process average temperature increases so does the quantity of small particles in the resultant tobacco product. This increase in small particles will lower the efficiency of the subsequent manufacturing process and increase the wastage of tobacco by increasing the quantity of dust removed.
It is the current expert view that tobacco cellular expansion results from an increase of water vapour pressure within the cell. One form of process equipment to achieve cellular expansion in this way is given in Patent GB2138666 in which a substantially horizontal vibrating tunnel is used to convey tobacco and steam is emitted from the base to the interior of the tunnel and passes through the transporting tobacco. That patent indicates average tobacco temperatures of 100.5.degree. C. to 120.degree. C. resulting from the use of steam at 2.5 to 25 bar and at steam temperature of 126.degree. C. to 400.degree. C.
In this apparatus steam is emitted into the tunnel in comparatively widely spaced streamlets and in practise the apparatus is operated typically with 3 to 7 bar pressure. For a tunnel 2.0 meter long by 0.4 meter wide GB2138666 utilises 7 rows of 15 holes per row and 0.8 mm diameter.
In operation an average product temperature of about 105.degree. C. results from the use of steam at 5 bar having a temperature of 152.degree. C. In practice, however, some particles of tobacco attain close to the steam temperature ie, 152.degree. C. while other particles experience fewer contacts with the steam streamlets and will only reach lower temperatures.
In consequence the resultant average tobacco temperature of 105.degree. C. is made up of particles with temperatures below 105.degree. C. and other particles with temperatures of up to 152.degree. C.
Particles which have not received sufficient heat will experience lower than average cellular expansion, while particles which have reached higher than average temperatures will have an increased fragility and be more likely to size degrees during subsequent handling as was illustrated in the table above.
The disadvantages of GB2138666 are partially alleviated by U.S. Pat. No. 5,161,548 which uses steam pressure and a far greater number of steam streamlets. U.S. Pat. No. 5,161,548 typically uses 5,000 steam streamlets where GB2138666 would use 105 streamlets. However, in both cases the treatment gas is steam which has in relation to it's mass a level of volume, temperature and heat which is determined by it's pressure.
Consequently the use of GB2138666 or U.S. Pat. No. 5,161,548 to give an average tobacco temperature of say 70.degree. C. still subjects some of the tobacco particles to steam at 100.degree. C. since this is the lowest temperature of steam at normal atmospheric pressure.
A further application of this current invention is in conjunction with a metering tube as disclosed in GB1559507. In GB1559507 tobacco is passed down a substantially vertical metering tube or column. The tube is arranged to have a band of perforations running around it's diameter. Steam is passed through the perforations to heat and moisten the tobacco flowing through the tube. Process apparatus of this form may be used as part of a tobacco cellular expansion process or as a conditioning process. A common application is to condition rejected cigarettes prior to their entry into a separate machine which recovers tobacco from the cigarettes so that the tobacco can be re-used. It is important that the cigarettes at entry to the reclaim have sufficient moisture content to minimise the tobacco damage occurring during the reclaim operation.
Typically reject cigarettes will have a moisture content of 8 to 14% while the desirable moisture at entry to the reclaim plant is 16 to 18%. Hence there is a requirement to add a controlled amount of water to give a moisture rise of 2 to 10% and also to operate at as lower temperature as possible in order to minimise temperature induced changes to the tobacco's chemical and physical properties.
As steam flows over the cigarettes it will lose heat and moisture by condensation which in turn raises the temperature and moisture content of the cigarette. This process continues until the cigarette reaches the steam temperature.
As the condensation occurs and removes moisture from the steam, the steam volume decreases. This means that, considering the metering tube example, the cigarettes close to the steam entry perforations must reach close to the steam temperature before steam can flow past them to condition other cigarettes.
A frequently met practical consequence is that at the tube discharge cigarettes near the periphery of the tube are hot and have gained moisture while those that flowed down the centre of the tube may be cool and have received very little moisture gain.
The moisture gained by these cigarettes in contact with the steam is dependent on their specific heat and initial temperature. This gain can be calculated to be usually in the range of 2.5 to 5.0% compared to the desired gain of 2 to 10%. Further, once the cigarettes have left the tube, they will start to experience evaporative cooling and the moisture content of the cigarette will reduce. A typical evaporative cooling loss is about 1.0%.
For a cigarette input moisture to the tube of 8% the expected moisture at the entry to cigarette reclaim becomes 9.5 to 12% or for tube entry moisture of 14% the reclaim entry expected moisture becomes 15.5 to 18% compared to the desired 16 to 18%. Consequently a large proportion of the input cigarettes are at risk of being conditioned to below the desired moisture, and those cigarettes which have been conditioned have also been subjected to detrimental temperatures.
The moisture gain of tobacco from steam is limited by temperature balance and ceases when the tobacco and steam reach the same temperature. The moisture gain of tobacco from a gas which is a mixture of air and water vapor is limited by vapor pressure balance. Moisture will continue to transfer from the air to the tobacco until the vapor pressure of water in the tobacco equals the vapor pressure of the water air mixture. This is illustrated by the fact that tobacco left in an environment of 22.degree. C. 75% relative humidity can eventually reach equilibrium moistures of 25 to 30% irrespective of their starting moisture.
Consequently if a conditioning metering tube is supplied with a gas made up of a mixture of air and water vapor greater tobacco moisture increased can be obtained at lower gas and tobacco temperatures then would result from the use of steam.
The vapor pressure, temperature, volume and heat content of the gas can be pre-determined by mixing controllable quantities of air, steam water spray in a mixing chamber which can contain additional heating elements. That prepared gas mixture is then supplied to a suitable process machine for application to the tobacco.
It is now being realised, however, that subjecting certain types of tobacco to temperatures in excess of 100.degree. C. or more can damage the tobacco structure, natural soluble or volatile organic compounds can be driven off, and, in general, the character of the tobacco can be diminished.
One method of treating tobacco which does not involve high temperatures comprises the intensive soaking of tobacco rib material in water. This is a well accepted method of treating tobacco. Heat is absorbed either simultaneously or subsequently to enable the ribs to expand.
Whilst this treatment is relatively gentle, a secondary treatment comprising rapid drying of the exterior whilst retaining the moisture within the rib is also required. A further problem encountered with water soaking is that the resulting product can be objectively sticky since resinous water extracted solids tend to remain on the surface of the tobacco. This sort of treatment can also damage the tobacco structure, can remove water soluble compounds and the character of the tobacco can be diminished.